The present invention relates generally to a ferrous sulfide suspension for the treatment and removal of selenium from industrial gases.
The emission of pollutants from coal-fired boilers is a major environmental concern. These pollutants may enter the environment in gaseous form (e.g. stack emissions), liquid form (e.g. discharges of treated or untreated wastewater effluents), or in the solid form (e.g. coal combustion byproducts).
Release of selenium to the environment, in particular as a pollutant contained in wastewater effluent discharges, is of particular concern.
Although selenium is an essential element in human and animal nutrition, selenium may become toxic if consumed in high levels. Therefore the concentration and total quantity of selenium in wastewater effluent discharges from coal-fired boilers, mining activities, and industrial sources is subject to strict government regulation.
Selenium is normally present in aqueous media such as wastewater effluents in the form of “selenite” ions and “selenate” ions. For the purposes of this disclosure, the terms “SeO32−”, “Se4+”, or “tetravalent selenium” have been used interchangeably when referring to the selenite ion, while the terms “SeO42−”, “Se6+”, or “hexavalent selenium” have been used interchangeably when referring to the selenate ion.
Certain pollutants (e.g. mercury), may be captured and removed from industrial gases by injection of a dry sorbent into the industrial gas stream with subsequent collection of the sorbent in a particulate matter control device such as an electrostatic precipitator or a fabric filter. These systems are collectively referred to as “dry scrubber” systems. Of the known dry sorbents, activated carbon and calcium-based sorbents have been the most actively studied and most widely used on a commercial basis.
Currently, the most commonly used sorbent in dry scrubber systems for mercury emission control in coal-fired and oil-fired plants is the injection of powdered activated carbon (PAC) into the flue gas stream.
Examples of other sorbents that have been used for mercury removal in dry scrubber systems include those disclosed in U.S. Patent Application Publication No. 2003/0103882 to Biermann et al and in U.S. Pat. No. 6,719,828 to Lovell et al. which discloses the preparation of layered sorbents such as clays with metal sulfides interlayered between the clay layers. Other patents for mercury capture by injection of dry sorbents are based upon preparation of the sorbents by thinly layering a chemical compound onto or into a substrate. These types of sorbents use substrates that include sol-gel derivatives as disclosed in U.S. Pat. No. 7,790,830 to Edmiston; U.S. Pat. No. 8,119,759 to Edmiston; and U.S. Pat. No. 8,217,131 to Edmiston, self-assembled monolayers on mesoporous supports as disclosed in U.S. Pat. No. 8,088,283 to Pate; U.S. Pat. No. 8,412,664 to Shankle; and U.S. Pat. No. 8,197,687 to Krogue et al., phyllosilicates as disclosed in U.S. Pat. No. 7,288,499 to Lovell et al., or variety of other substrates. In addition to these U.S. Pat. No. 7,575,629 to Yang et al. and U.S. Pat. No. 7,704,920 to Yang et al. disclose that any metal salt that can release a metal ion when the salt contacts a sulfide salt forming a water insoluble metal sulfide on the substrate surface can be used to produce an effective dry sorbent for mercury removal.
The aforementioned dry sorbents, which are complex and expensive to produce and use, may be effective for the removal of certain pollutants or contaminants from industrial gases, however these dry sorbents have not been shown to be effective in the removal of selenium from industrial gases.
Another type of scrubber system that is used to lower the emission of toxic gaseous pollutants or other contaminants to the environment are commonly referred to as “wet scrubbers”. In a wet scrubber system, industrial gases containing pollutants or contaminants are brought into contact with a scrubbing liquid or slurry (scrubber liquor) to create a gas-liquid interface to transfer the pollutants or contaminants from the industrial gases into the scrubber liquor, either by (1) spraying the industrial gases containing pollutants or contaminants with the scrubber liquor, (2) by dispersing or forcing the industrial gases through the scrubber liquor, or (3) by any other means to transfer the pollutants or contaminants from the industrial gases into the scrubber liquor.
The composition of the scrubber liquors used in these wet scrubber systems varies depending upon the pollutants or contaminants in the industrial gases targeted for removal. For example, in a wet flue gas desulfurization device used to remove acids (e.g. HCl, HSO3) or sulfur dioxide (SO2), a slurry liquor containing limestone (CaCO3), oxides or hydroxides of calcium or magnesium, or other mixtures are primarily used.
Although a particular wet scrubber system may be designed and operated to remove one or more targeted pollutants or contaminants, it may also concurrently remove other pollutants or contaminants from industrial gases to varying degrees of effectiveness. One of these pollutants or contaminants is selenium.
Since the environment in a wet scrubber system is dynamic, removal of any pollutant or contaminant from an industrial gas is complex. Successful removal of pollutants or contaminants from the industrial gas must account for the various equilibrium conditions present between the pollutants or contaminants in the industrial gas prior to entering the wet scrubber system.
Once the industrial gas containing pollutants or contaminants enters the wet scrubber system, the successful removal of these pollutants or contaminants from the industrial gas is predicated on concurrently controlling the physical and chemical reactions that are a result of interactions between the pollutants or contaminants in the industrial gas and the solid, liquid, and gaseous phases present (or created) in the entire wet scrubber system, and in particular the wet scrubber liquor itself.
Once selenium is removed from the industrial gas and the selenium is now in the wet scrubber liquor, removal of the selenium from the wet scrubber liquor is further complicated since the physical, chemical, and oxidation-reduction conditions required to remove the selenium from the wet scrubber liquor may be different (or diametrically opposed) to those conditions required to treat or remove other target pollutants or contaminants contained in the wet scrubber liquor.
One complicating factor specific to the treatment or removal of selenium from scrubber liquors is the selenium species in wet scrubber liquors are predominantly selenate ions or selenate complexes which are more difficult and expensive to remove from the scrubber liquors.
In general, once scrubber liquors are no longer effective or useful for their intended purpose, the scrubber liquors are treated to lower the concentration or total mass loading of any pollutants or contaminants contained within the scrubber liquor and the treated wet scrubber liquor is discharged from the scrubber liquor wastewater treatment system into the environment.
One removal mechanism of pollutants or contaminants from industrial gas streams that become dissolved in the aqueous phase of wet scrubber liquors in wet scrubber systems relies upon absorption of the “ionic” or “oxidized” phase of the pollutant or contaminant. For example, a metal cation (“M”) in the industrial gas phase may dissolve in the wet scrubber liquor, and if an oppositely charged anion (“X”) is also present in wet scrubber liquor form an “MX” complex or MX precipitate. In these cases, absorption is a phenomenon whereby atoms, molecules, or ions that are present in the industrial gas stream are absorbed (taken up) by the volume of the bulk (liquid) phase.
Another removal mechanism is “adsorption” which is a physical phenomenon where atoms, ions, or molecules from a gas, liquid, or dissolved solid adhere (bind) to another solid surface. The exact nature of the bonding by adsorption is dependent upon the species involved, but the adsorption process is generally classified as physisorption (characteristic of weak van der Waals forces), chemisorption (characteristic of covalent bonding), or some other type of electrostatic attraction. In other words, absorption is the process through which a substance, originally present in one phase, is removed from that phase by dissolution into another phase (typically a liquid), as opposed adsorption which is the accumulation of atoms, ions, or molecules from a bulk liquid or gas onto a solid surface.
A variety of treatment technologies, including reverse osmosis, ion exchange, coagulation, adsorption, and biological treatment, have been applied in order to remove selenium from aqueous media (e.g. wastewaters, scrubber liquors). Among them, adsorption using Fe-, Mn-, or Al-(oxy)hydroxides has been extensively studied because adsorption of aqueous selenium species onto such mineral surfaces plays an important role in determining the mobility and bioavailability of selenium. Although these treatment methods may be able to lower both Se2− and Se4+ to below 5 μg/L, they are not suitable for treatment or removal of Se6+ in wastewaters originating at coal-fired power plants or other industrial activities also containing high concentration of sulfate ions ((SO4(2−))) since the physio-chemical properties of sulfate ions are similar to that of Se(+6), and as a result, the competitive adsorption of sulfate ions significantly decreases the removal efficiency of Se(+6) from these types of wastewaters.
The present invention overcomes the disadvantage of removal of selenium from industrial gases by (1) sorbents in dry scrubbers systems which are primarily based on adsorption onto the sorbent, and (2) treatment or removal of selenium containing wet scrubber liquors wastewaters prior to discharge to the environment.